Vulcanizable rubber compositions inhibited from premature vulcanization by methylene thioethers

ABSTRACT

Vulcanizable rubber compositions are described which are inhibited from premature vulcanization by activated methylene di(thioethers) in which the adjacent activating groups are either carbonyl or cyano. Novel compounds are also described.

This invention relates to improved vulcanizable rubber compositions inhibited from premature vulcanization by active methylene thioethers and to certain novel active methylene thioethers.

BACKGROUND OF THE INVENTION

Sulfenamide inhibitors are especially potent for vulcanizable rubber compositions containing sulfenamide accelerators but are less effective when the accelerator is 2-mercaptobenzothiazole or bis-2-benzothiazolyl disulfide. Acid retarders reduce scorchiness of benzothiazole accelerated stocks by adversely affecting both the cure rate and the extent of cure. A study of the mechanism of retarders in rubber vulcanization lead to a class of retarders consisting of hydrocarbylthio derivatives of trialkylsulfonyl methane, J. Appl. Poly. Sci., Vol. 16, pages 2647-2655 (1972), however, these retarders have not gained acceptance. Therefore, there is a need for more effective inhibitors for benzothiazole accelerated stocks.

SUMMARY OF THE INVENTION

It has now been discovered that vulcanizable rubber compositions are protected from premature vulcanization by di(thioethers) derived from activated methylene compounds in which the adjacent activating groups are either carbonyl or cyano. Thus, the inhibitors of the invention are either α,α-gem-disulfenated ketones or α,α-gem-disulfenated nitriles. The two activating groups adjacent to the methylene group may be two carbonyl groups, two cyano groups, or one carbonyl group and one cyano group.

Vulcanizable compositions of the invention comprise sulfur-vulcanizable rubber, sulfur-vulcanizing agent, organic vulcanization accelerating agent and, in an amount effective to inhibit premature vulcanization, a di(thioether) of the formulas ##STR1## in which R is alkyl of 1-20 carbon atoms, cycloalkyl of 5-12 carbon atoms, benzyl, α-methylbenzyl, phenyl, or substituted phenyl wherein the substituents are lower alkyl, lower alkoxy, or halo, R₁ is R or anilino or substituted anilino wherein the substituents are lower alkyl, lower alkoxy or halo, R₂ is phenyl or substituted phenyl as above, ##STR2## in which R is the same as above and X is o-phenylene or straight or branched alkylene of 2-4 chain carbon atoms, ##STR3## in which R is the same as above, R₃ is lower alkyl or lower alkoxy, and R₄ is carboalkoxy of 1-5 carbon atoms or ##STR4## or R₄ may be lower alkyl provided that the vulcanization accelerating agent is 2-mercaptobenzothiazole, and ##STR5## in which R is the same as above and R₅ is cyano, carboalkoxy of 1-5 carbon atoms, substituted carbamoyl of the formula ##STR6## in which R₆ is hydrogen or R, R₇ is R or carboalkoxy of 1-5 carbon atoms, or R₆ and R₇ together is alkylene or oxydialkylene of 4-8 carbon atoms which along with the nitrogen atoms forms a heterocycle, or R₅ may be carbamoyl provided that the vulcanization accelerating agent is 2-mercaptobenzothiazole.

As indicated by the above provisos, inhibitors of Formula (III) wherein R₄ is lower alkyl and inhibitors of Formula (IV) wherein R₅ is unsubstituted carbamoyl are potent inhibitors when the accelerating agent is 2-mercaptobenzothiazole but are less effective and require high loadings to inhibit premature vulcanization of compositions containing other accelerators, for example, sulfenamide accelerators. This contrasts with the inhibitors of Formula (I) and (II) and the remaining inhibitors of Formulas (III) and (IV) which exhibit inhibitor activity at conventional loadings with all ordinary organic accelerating agents for sulfur vulcanization.

The term "lower" when referring to alkyl or alkoxy means a radical containing 1-5 carbon atoms. Preferred inhibitors of Formula (I) are derived from acetoacetanilide or α-benzoylacetanilide. Preferred inhibitors of Formula (II) are derived from 1,3-cyclohexanedione or 5,5-dialkyl-1,3-cyclohexanedione. More preferred inhibitors are compounds in which R is phenyl. Preferred inhibitors of Formula (IV) are compounds in which R₅ is cyano, carbethoxy, carbomethoxy, carbethoxycarbamoyl or carbomethoxycarbamoyl.

Examples of satisfactory radicals for R are methyl, ethyl, propyl, isopropyl, n-butyl, sec.-butyl, isobutyl, pentyl, sec.-pentyl (1-methylbutyl), hexyl, heptyl, octyl, nonyl, decyl, dodecyl, eicosyl, cyclopentyl, cyclohexyl, 2-methylcyclohexyl, 4-methylcyclohexyl, cycloheptyl, cyclooctyl, cyclodecyl, cyclododecyl, 2-methylphenyl, 3-methylphenyl, 4-methylphenyl, 2-methyl-4-t-butylphenyl, 4-t-butylphenyl, 2,4,6-trimethylphenyl, 3-isopropylphenyl, 4-methoxyphenyl, 4-ethoxyphenyl, 4-chlorophenyl, and 2,4-dichlorophenyl. Hydrocarbon radicals are preferred. Preferred alkyl radicals are primary or secondary lower alkyl radicals of 1-5 carbon atoms.

Examples of satisfactory radicals for ##STR7## are methylamino, ethylamino, propylamino, isopropylamino, butylamino, t-butylamino, cyclohexylamino, benzylamino, anilino, dimethylamino, diethylamino, dipropylamino, dicyclohexylamino, diphenylamino, pyrrolidinyl, piperidino, morpholino, 2,6-dimethylmorpholino, and hexahydro-1H-azepine-1-yl.

The inhibitors of the invention are incorporated into rubber stocks by mixing on a mill or in an internal mixer such as a Banbury mixer. However, the inhibitors may be incorporated by addition to latex, if desired. The process of the invention is particularly applicable to sulfur-vulcanizable rubber compositions containing a sulfur vulcanizing agent such as an amine disulfide or a polymeric polysulfide but preferably, the vulcanizing agent is elemental sulfur (usually about 0.5-5 parts by weight of sulfur are used per 100 parts by weight of rubber). Rubber compositions containing organic accelerating agents are particularly improved by the inhibitors of the invention with compositions containing 2-mercaptobenzothiazole accelerator being preferred. Any organic accelerating agent in an amount effective (generally about 0.1-5 parts by weight accelerator per 100 parts by weight rubber) to accelerate the sulfur vulcanization of rubber is satisfactory in the practice of this invention. The inhibitors of the invention are effective with any sulfur-vulcanizable natural and synthetic rubber and mixtures thereof. Suitable accelerators and rubbers are described in U.S. Pat. No. 3,546,185, Col, 9, lines 53-75, Col. 10, lines 15-21, and U.S. Pat. No. 3,780,001, Col. 4, lines 43-72, Col. 5, lines 5-33, respectively, the disclosures of which are incorporated herein by reference. The vulcanizable composition may also contain conventional compounding ingredients such as reinforcing pigments, extenders, processing oils, antidegradants and the like.

Small amounts of inhibitors are effective to inhibit premature vulcanization. Improvements in processing safety may be observed with 0.05 parts or less of inhibitor per 100 parts rubber. Although there is no upper limit in the amount of inhibitor used, generally the amount does not exceed 5 parts inhibitor per 100 parts rubber. Typically, the amount of inhibitor added is about 0.1 to 2.5 parts per 100 parts rubber with amounts of about 0.2 to 1 parts of inhibitor per 100 parts rubber being commonly used. Methods for determining scorch times and curing characteristics of rubber stocks used in illustrating this invention are described in U.S. Pat. No. 3,546,185, Col. 13, lines 30-53. Stress-strain properties are reported in megapascals (MPa).

One embodiment of the invention concerns novel compounds of the formulas ##STR8## in which R is alkyl of 1-20 carbon atoms, cycloalkyl of 5-12 carbon atoms, benzyl, α-methylbenzyl, phenyl, or substituted phenyl wherein the substituents are lower alkyl, lower alkoxy, or halo, R₁ is R or anilino or substituted anilino wherein the substituents are lower alkyl, lower alkoxy or halo, R₂ is phenyl or substituted phenyl as above, ##STR9## in which R is the same as above and X is o-phenylene or straight or branched alkylene of 2-4 chain carbon atoms, ##STR10## in which R is the same as above, R₃ is lower alkyl or lower alkoxy and R₄ is carboalkoxy of 1-5 carbon atoms or ##STR11## in which R is the same as above and R₅ is carboalkoxy of 1-5 carbon atoms, substituted carbamoyl of the formula ##STR12## in which R₆ is hydrogen or R, R₇ is R or carboalkoxy of 1-5 carbon atoms, or R₆ and R₇ together is alkylene or oxydialkylene of 4-8 carbon atoms which along with the nitrogen atoms forms a heterocycle. In preferred compounds of Formula (V), R₁ is methyl or phenyl and R₂ is phenyl. In preferred compounds of Formula (VIII), R₅ is carboalkoxy or carboalkoxycarbamoyl. In more preferred compounds R is phenyl.

Certain inhibitors of the invention are known, for example, 3,3-diphenylthio-2,4-pentanedione and 2,2-diphenylthiomalononitrile are described in Bulletin of the Chemical Society of Japan, Vol. 45, 866-870 (1972). The synthesis methods described therein are suitable for preparing inhibitors of this invention. Alternatively, the inhibitors of the invention may be prepared by reacting a sulfenyl chloride with the appropriate active methylene compound. Preferably, the reaction is carried out in the presence of an acid acceptor such as triethylamine, pyridine, and piperidine. Examples of suitable active methylene compounds useful as intermediates are acetoacetanilide (N-phenyl-3-oxobutaneamide), benzoylacetanilide, bis(phenylcarbamoyl)methane, 1,3-cyclohexanedione, indandione, methylacetoacetate, diethyloxalacetate or sodium salt thereof, 2-cyanoacetamide, ethyl cyanoacetate, N-cyanoacetylurethane (ethyl-N-cyanoacetylcarbamate), N-methyl-2-cyanoacetamide, and 2-cyanoacetanilide. Other suitable intermediates are 1-cyanoacetylpyrrolidine, N-phenyl-3-oxo-pentaneamide, 2,4-pentanedione, 2,4-hexanedione, 3,5-heptanedione, 1,3-cyclopentanedione, 1,3-cycloheptanedione, 4-methylindandione, 5-methylindandione, ethylacetoacetate, propylacetoacetate, dimethyloxalacetate (dimethyl-2-oxo-butanedioate), dipropyloxalacetate, methyl cyanoacetate, propyl cyanoacetate, N-cyclohexylcyanoacetamide, 4-(cyanoacetyl)morpholine, N,N-dimethylcyanoacetamide, N,N-diphenylcyanoacetamide, N-methyl-N-phenylcyanoacetamide, 1-cyanoacetylpiperidine, methyl N-cyanoacetylcarbamate, propyl N-cyanoacetylcarbamate, bis(N-methylcarbamoyl)methane and dimethyl 1,3-acetonedicarboxylate.

Examples of inhibitors of the invention are α-acetyl-α,α-di(benzylthio)acetanilide, α-acetyl-α,α-di(butylthio)acetanilide, α-acetyl-α,α-di(cyclohexylthio)acetanilide, bis (phenylcarbamoyl)bis(isopropylthio)methane, bis(phenylcarbamoyl)bis(cyclohexylthio)methane, 2,2-di(phenylthio)-1,3-cyclopentanedione, 2,2-di(phenylthio)-1,3-cycloheptanedione, 2,2-di(isopropylthio)-1,3-cyclopentanedione, 2,2-di(isopropylthio)-1,3-cycloheptanedione, 2,2-di(cyclohexylthio)-indandione, 2,2-di(isopropylthio)-indandione, 2,2-di(phenylthio)-N-phenyl-3-oxo-pentaneamide, 2,2-di(isopropylthio)-N-phenyl-3-oxo-pentaneamide, 2,2-di(cyclohexylthio)-N-phenyl-3-oxopentaneamide, 2,2-di(benzylthio)-1,3-cyclohexanedione, bis (phenylcarbamoyl)bis(benzylthio)methane, 3,3-di(isopropylthio)-2,4-pentanedione, 3,3-di(phenylthio)-2,4-hexanedione, 4,4-di(phenylthio)-3,5-heptanedione, 2,2-di(benzylthio)indandione, 2,2-di(phenylthio)-4-methylindandione, 2,2-di(phenylthio)-5-methylindandione, ethyl α-acetyl-α,α-di(isopropylthio) acetate, propyl α-acetyl-α,α-di(isopropylthio)acetate, dimethyl 2,2-di(phenylthio)oxalacetate, dipropyl 2,2-di(phenylthio)oxalacetate, bis(phenylthio)bis(N-methylcarbamoyl)methane, bis(benzylthio)bis(N-methylcarbamoyl)methane, bis(isopropylthio)bis(N-methylcarbamoyl)methane, bis(cyclohexylthio)bis(N-methylcarbamoyl)methane, methyl di(benzylthio)cyanoacetate methyl di(isopropylthio)cyanoacetate, methyl di(cyclohexylthio)cyanoacetamide, 4-(1,1-di[phenylthio]-1-cyanoacetyl)morpholine, N,N-dimethyl-2,2-di(phenylthio)cyanoacetamide, N,N-diphenyl-2,2-di(isopropylthio)cyanoacetamide, N-methyl-N-phenyl-2,2-di(phenylthio)cyanoacetamide, methyl di(phenylthio)cyanoacetylcarbamate, methyl di(isopropylthio)-N-cyanoacetylcarbamate and dimethyl 1,1,3,3-tetra(isopropylthio)-1,3-acetonedicarboxylate.

PREFERRED EMBODIMENTS

Examples of inhibitors of the invention are prepared by the following typical procedure. A sulfenyl chloride solution is first prepared by conventional procedures by chlorination at about 0° C. of a mercaptan in an appropriate solvent, for example, methylene chloride. A solution containing two molecular equivalents of a sulfenyl chloride is then added at 0°-25° C. with stirring to a solution containing one molecular equivalent of an active methylene compound, 2.2 molecular equivalents of an acid acceptor, for example, triethylamine, and an appropriate solvent, for example, methylene chloride. The reaction mixture is then stirred for 30-60 minutes. Recovery and purification of the product are conventional. The reaction mixture is then washed with water to remove amine salt by-product. In the case of insoluble products, the solid product is then recovered by filtration. In the case of soluble products, the organic layer is dried over sodium sulfate, filtered and the filtrate is evaporated to recover the product. The product is then recrystallized from an appropriate solvent, for example, ethanol. Identification of the products is confirmed by one or more analyses consisting of elemental, infrared and nuclear magnetic spectral analyses.

Compounds prepared in this manner are tabulated as examples 1-32 in Tables 1-4.

                                      TABLE 1                                      __________________________________________________________________________      ##STR13##                                                                     Example                                  Sulfur Analysis                       No.  Compound      R     R.sub.1                                                                            R.sub.2                                                                              m.p. °C.                                                                      Calc.                                                                              Found                             __________________________________________________________________________     1    α-acetyl-α,α-di(phenylethyl)                                               phenyl                                                                               methyl                                                                             phenyl                                                                               172-173                                                                              15.51                                                                              16.29                                  acetanilide                                                               2    α-acetyl-α,α-di(cyclohexyl-                                                cyclohexyl                                                                           methyl                                                                             phenyl                                                                                 158-159.5                                                                          15.24                                                                              15.81                                  thio)acetanilide                                                          3    α-acetyl-α,α-di(isopropyl-                                                 isopropyl                                                                            methyl                                                                             phenyl                                                                               118-119                                                                              19.76                                                                              19.70                                  thio)acetanilide                                                          4    α-acetyl-α,α-di(methyl-                                                    methyl                                                                               methyl                                                                             phenyl                                                                               114-115                                                                              23.50                                                                              23.80                                  thio)acetanilide                                                          5    α-acetyl-α,α-di(ethylthio)                                                 ethyl methyl                                                                             phenyl                                                                               72-73 --  --                                     acetanilide                                                               6    α-acetyl-α,α-di(propyl-                                                    propyl                                                                               methyl                                                                             phenyl                                                                               83    19.26                                                                              19.70                                  thio)acetanilide                                                          7    α-acetyl-α,α-di(phenyl-                                                    phenyl                                                                               methyl                                                                             2-methoxy-                                                                           68-73 --  --                                     thio)-o-acetanisidide   phenyl                                            8    α-acetyl-α,α-di(cyclohexyl-                                                cyclohexyl                                                                           methyl                                                                             4-chloro-                                                                            146-147                                                                              --  --                                     thio)-p-chloroacetanilide                                                                              phenyl                                            9    α-acetyl-α,α-di(cyclohexyl-                                                cyclohexyl                                                                           methyl                                                                             4-chloro-                                                                            176-177                                                                              --  --                                     thio)-p-chloroacetanilide                                                                              phenyl                                            benzoyl-α,α-di(iso-                                                     isopropyl     phenyl                                                                               phenyl                                                                             137-138                                                                              16.55 16.35                                      propylthio)acetanilide                                                    benzoyl-α,α-di(phenyl-                                                  phenyl        phenyl                                                                               phenyl                                                                             226-228                                                                              13.41 14.07                                      thio)acetanilide                                                          12   bis(phenylcarbamoyl)bis                                                                      phenyl                                                                               anilino                                                                            phenyl                                                                               198-199                                                                              --  --                                     (phenylthio)methane                                                       __________________________________________________________________________

                                      TABLE 2                                      __________________________________________________________________________      ##STR14##                                                                     Example                              Sulfur Analysis                           No.  Compound     R     X      m.p. ° C.                                                                     Calc.                                                                              Found                                 __________________________________________________________________________     13   2,2-di(phenylthio)-1,3-                                                                     phenyl                                                                               1,3-propylene                                                                         126-127                                                                              19.52                                                                              19.55                                      cyclohexanedione                                                          14   2,2-di(cyclohexylthio)-                                                                     cyclohexyl                                                                           1,3-propylene                                                                         170.5-172                                                                            18.83                                                                              18.29                                      1,3-cyclohexanedione                                                      15   2,2-di(isopropylthio)-                                                                      isopropyl                                                                            1,3-propylene                                                                         157-158                                                                              --  --                                         1,3-cyclohexanedione                                                      16   2,2-di(phenylthio)-5,5-                                                                     phenyl                                                                               2,2-dimethyl-                                                                         160.5-162                                                                            17.99                                                                              17.69                                      dimethyl-1,3-cyclohexane-                                                                         1,3-propylene                                               dione                                                                     17   2,2-di(cyclohexylthio)-                                                                     cyclohexyl                                                                           2,2-dimethyl-                                                                         191-193                                                                              17.40                                                                              16.79                                      5,5-dimethyl-1,3-cyclo-                                                                           1,3-propylene                                               hexanedione                                                               18   2,2-di(phenylthio)-                                                                         phenyl                                                                               o-phenylene                                                                           126-128                                                                              --  --                                         indandione                                                                __________________________________________________________________________

                                      TABLE 3                                      __________________________________________________________________________      ##STR15##                                                                     Example                                                                        No.  Compound      R     R.sub.3                                                                             R.sub.4 m.p. ° C.                         __________________________________________________________________________     19   diethyl 2,2-di(phenylthio)-                                                                  phenyl                                                                               ethoxy                                                                              carbethoxy                                                                             74-76                                         oxalacetate                                                               20   diethyl 2,2-di(isopropyl-                                                                    isopropyl                                                                            ethoxy                                                                              carbethoxy                                                                             liquid                                        thio)oxalacetate                                                          21   3,3-di(phenylthio)-2,4-                                                                      phenyl                                                                               methyl                                                                              methyl  119-120                                       pentanedione                                                              22   methyl α,α-di(phenylthio)-                                                       phenyl                                                                               methoxy                                                                             methyl  75.5-76                                       acetoacetate                                                              23   diethyl 1,1,3,3-tetra- (phenylthio)-1,3-acetone- dicarboxylate                               phenyl                                                                               ethoxy                                                                               ##STR16##                                                                             145-146                                  24   diethyl 1,1,3,3-tetra- (cyclohexylthio)-1,3- acetonedicarboxylate                            cyclohexyl                                                                           ethoxy                                                                               ##STR17##                                                                             liquid (70% assay)                       __________________________________________________________________________

                                      TABLE 4                                      __________________________________________________________________________      ##STR18##                                                                     Example                              Sulfur Analysis                           No.  Compound     R    R.sub.5  m.p. °C.                                                                     Calc.                                                                              Found                                 __________________________________________________________________________     25   2,2-di(phenylthio)-2-                                                                       phenyl                                                                              carbamoyl                                                                               151-152                                                                             21.58                                                                              21.35                                      cyanoacetamide                                                            26   N-(α,α-[diphenylthio]                                                           phenyl                                                                              carbethoxy-                                                                             113-114                                                                             16.59                                                                              17.22                                      cyanoacetyl)urethane                                                                             carbamoyl                                               27   ethyl-di(phenylthio)-                                                                       phenyl                                                                              carbethoxy                                                                              36   19.36                                                                              19.46                                      cyanoacetate                                                              28   2,2-di(phenylthio)-                                                                         phenyl                                                                              cyano    54-55                                                                               --  --                                         malononitrile                                                             29   1-(α-cyano-α,α-di[phenyl-                                                 phenyl                                                                              pyrrolidinyl-                                                                           86   17.42                                                                              18.14                                      thio]acetyl)pyrrolidine                                                                          carbonyl                                                30   2,2-di(isopropylthio)-                                                                      isopropyl                                                                           carbamoyl                                                                               75   --  --                                         2-cyanoacetamide                                                          31   2,2-di(phenylthio)-2-                                                                       phenyl                                                                              phenylcarbamoyl                                                                         110  --  --                                         cyanoacetanilide                                                          32   2,2-di(isopropylthio)-                                                                      isopropyl                                                                           phenylcarbamoyl                                                                         69-70                                                                               --  --                                         2-cyanoacetanilide                                                        __________________________________________________________________________

The invention is illustrated by incorporating inhibitors of the invention into the following natural rubber masterbatch.

    ______________________________________                                                       Masterbatch                                                      ______________________________________                                         Smoked Sheets   100                                                            Carbon Black    40                                                             Extender Oil    10                                                             Wax             2                                                              Zinc Oxide      5                                                              Stearic Acid    1                                                                              158                                                            ______________________________________                                    

Vulcanizable compositions are prepared by incorporation of accelerator and sulfur. Compositions of the invention are prepared by incorporating inhibitors in the indicated quantities into the vulcanizable compositions. Compositions containing no inhibitors are controls. The results are shown in Tables 5-12.

In Table 5, the vulcanizable rubber composition comprises 158 parts of natural rubber masterbatch, 2 parts of N-(1,3-dimethylbutyl)-N'-(phenyl)-p-phenylenediamine (Santoflex 13 antidegradant), 0.5 parts of N-(t-butyl)benzothiazolesulfenamide (Santocure NS accelerator) and 2.5 parts of sulfur. All parts are by weight. Stock 1 is a control containing no inhibitor. Stocks 2-10 contain 1 part of inhibitor.

The data show that α,α-gem-disulfenated derivatives of acetoacetanilide and benzoylacetanilide impart processing safety to the vulcanizable compositions as shown by the increase in scorch delay. The data further indicate that the phenyl and isopropyl compounds exhibit greater activity and that the benzoylacetanilide compound is especially potent.

                                      TABLE 5                                      __________________________________________________________________________                     1  2  3  4  5  6  7  8  9  10                                  __________________________________________________________________________     NR Masterbatch  158                                                                               →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                            Santoflex® 13 Antidegradant                                                                2  →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                            Santocure® NS Accelerator                                                                  0.5                                                                               →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                            Sulfur          2.5                                                                               →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                            α-acetyl-α,α-di(phenylthio)                                                  -- 1  -- -- -- -- -- -- -- --                                  acetanilide                                                                    α-acetyl-α,α-di(cyclohexyl-                                                  -- -- 1  -- -- -- -- -- -- --                                  thio)acetanilide                                                               α-acetyl-α,α-di(isopropyl-                                                   -- -- -- 1  -- -- -- -- -- --                                  thio)acetanilide                                                               α-acetyl-α,α-di(methylthio)                                                  -- -- -- -- 1  -- -- -- -- --                                  acetanilide                                                                    α-acetyl-α,α-di(propylthio)                                                  -- -- -- -- -- 1  -- -- -- --                                  acetanilide                                                                    α-acetyl-α,α-di(ethylthio)                                                   -- -- -- -- -- -- 1  -- -- --                                  acetanilide                                                                    α-acetyl-α,α-di(phenylthio)-p-                                               -- -- -- -- -- -- -- 1  -- --                                  chloroacetanilide                                                              α-acetyl-α,α-di(cyclohexylthio)-                                             -- -- -- -- -- -- -- -- 1  --                                  p-chloroacetanilide                                                            α-benzoyl-α,α-di(phenylthio)                                                 -- -- -- -- -- -- -- -- -- 1                                   acetanilide                                                                    MOONEY SCORCH @ 121° C.                                                 t.sub.5, minutes                                                                               28.6                                                                              51.6                                                                              44.4                                                                              53.9                                                                              38.0                                                                              41.0                                                                              42.5                                                                              48.4                                                                              45.5                                                                              68.4                                % increase in scorch delay                                                                     -- 80 55 88 33 43 49 69 59 139                                 STRESS-STRAIN @ 153° C.                                                 M.sub.300, MPa  7.5                                                                               7.3                                                                               6.9                                                                               6.7                                                                               7.3                                                                               6.7                                                                               6.7                                                                               7.1                                                                               7.3                                                                               6.4                                 UTS, MPa        24.9                                                                              25.8                                                                              24.6                                                                              25.1                                                                              26.6                                                                              25.6                                                                              25.9                                                                              25.6                                                                              25.2                                                                              25.3                                Elong., %       620                                                                               670                                                                               630                                                                               650                                                                               650                                                                               670                                                                               660                                                                               660                                                                               650                                                                               620                                 __________________________________________________________________________

Vulcanizable compositions containing 2-mercaptobenzothiazole as the vulcanization accelerator are illustrated in Tables 6 and 7. The data show that the inhibitors substantially enhance the scorch safety of the compositions and, in addition, increase the cure rate and the tensile strength. The data of Stocks 5-9 of Table 6 show that the amount of scorch delay is essentially directly proportional to the quantity of inhibitor used.

Vulcanizable rubber compositions containing different accelerators are shown in Table 8. The data indicate that the inhibitor of the invention is especially potent in the composition in which the accelerator is the amine salt of 2-mercaptobenzothiazole.

Inhibitors of the invention derived from cyclic diones are illustrated in Table 9. The data indicate the phenyl derivatives are more active.

Inhibitors of the invention derived from oxalacetate ester, acetoacetate ester, 2,4-pentanedione and indandione are illustrated in Table 10. The data show that the inhibitors exhibit substantial prevulcanization inhibitor activity in compositions in which the accelerator is 2-mercaptobenzothiazole. In other experiments, not shown, α,α-di(phenylthio) derivatives of diethyloxalacetate, 2,4-pentanedione, and methylacetoacetate at comparable loadings exhibit little or no activity in vulcanizable rubber compositions containing sulfenamide accelerators.

                                      TABLE 6                                      __________________________________________________________________________                   1  2  3  4  5  6  7  8  9                                        __________________________________________________________________________     NR Masterbatch                                                                               158                                                              2-mercaptobenzothiazole                                                                      1                                                                Sulfur        2.5                                                              α-acetyl-α,α-di(phenylthio)-                                               -- 0.855                                                                             -- -- -- -- -- -- --                                       p-chloroacetanilide                                                            α-acetyl-α,α-di(cyclohexyl-                                                -- -- 0.88                                                                              -- -- -- -- -- --                                       thio)-p-chloroacetanilide                                                      α-acetyl-α,α-di(methylthio)-                                               -- -- -- 0.54                                                                              -- -- -- -- --                                       acetanilide                                                                    α-acetyl-α,α-di(isopropyl-                                                 -- -- -- -- 0.65                                                                              1.3                                                                               -- -- --                                       thio)-acetanilide                                                              α-acetyl-α,α-di(phenylthio)-                                               -- -- -- -- -- -- 0.395                                                                             0.785                                                                             1.595                                    acetanilide                                                                    Mooney Scorch @ 121° C.                                                 t.sub.5, minutes                                                                             12.4                                                                              24.7                                                                              23.0                                                                              25.4                                                                              25.0                                                                              50.4                                                                              19.3                                                                              24.9                                                                              37.5                                     % increase in scorch                                                                         -- 99 85 105                                                                               102                                                                               306                                                                               56 101                                                                               202                                      delay                                                                          Rheometer @ 153° C.                                                     t.sub.90 -t.sub.2 , min.                                                                     9.6                                                                               8.3                                                                               8.5                                                                               10.1                                                                              8.8                                                                               9.0                                                                               8.5                                                                               7.9                                                                               8.9                                      Stress-Strain @ 153° C.                                                 M.sub.300, MPa                                                                               3.5                                                                               3.9                                                                               4.0                                                                               4.0                                                                               3.9                                                                               4.1                                                                               3.7                                                                               4.2                                                                               3.9                                      UTS, MPa      17.0                                                                              19.2                                                                              19.2                                                                              19.1                                                                              19.2                                                                              20.9                                                                              17.7                                                                              20.6                                                                              21.4                                     Elong., %     680                                                                               690                                                                               680                                                                               690                                                                               690                                                                               710                                                                               670                                                                               700                                                                               730                                      __________________________________________________________________________

                                      TABLE 7                                      __________________________________________________________________________                         1   2   3   4   5                                          __________________________________________________________________________     NR Masterbatch      158 158 158 158 158                                        2-mercaptobenzothiazole                                                                            1   1   1   1   1                                          Sulfur              2.5 2.5 2.5 2.5 2.5                                        α-benzoyl-α,α-di(phenylthio)acetanilide                                          --  0.91                                                                               --  --  --                                         α-acetyl-α,α-di(phenylthio)acetanilide                                           --  --  0.785                                                                              --  --                                         α-acetyl-α,α-di(cyclohexylthio)acetanilide                                       --  --  --  0.81                                                                               --                                         α-acetyl-α,α-di(phenylthio)-o-acetanisidide                                      --  --  --  --  0.865                                      Mooney Scorch @ 121° C.                                                 t.sub.5, minutes    13.5                                                                               26.1                                                                               23.7                                                                               24.9                                                                               21.5                                       % increase in scorch delay                                                                         --  93  76  84  59                                         Rheometer @ 153° C.                                                     t.sub.90 -t.sub.2, min.                                                                            9.5 8.5 7.9 8.2 8.1                                        Stress-Strain @ 153° C.                                                 M.sub.300, MPa      3.7 4.3 3.9 4.2 3.9                                        UTS, MPa            18.3                                                                               20.8                                                                               21.0                                                                               20.8                                                                               18.9                                       Elong., %           690 690 710 690 680                                        __________________________________________________________________________

                                      TABLE 8                                      __________________________________________________________________________                    1  2  3  4  5  6  7  8  9  10                                   __________________________________________________________________________     NR Masterbatch 158                                                                               →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                             Sulfur         2.5                                                                               →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                                                                          →                             bis-(dimethylthiocarbamyl)                                                                    0.5                                                                               0.5                                                                               -- -- -- -- -- -- -- --                                   sulfide                                                                        2,2'-bis(dithiobenzothiazole)                                                                 -- -- 1  1  -- -- -- -- -- --                                   zinc O,O'-dibutyl phosphoro-                                                                  -- -- -- -- 1.5                                                                               1.5                                                                               -- -- -- --                                   dithioate                                                                      zinc 2-mercaptobenzothiazole                                                                  -- -- -- -- -- -- 1  1  -- --                                   tert-butylamine salt of                                                                       -- -- -- -- -- -- -- -- 0.5                                                                               0.5                                  2-mercaptobenzothiazole                                                        α-acetyl-α,α-di(phenylthio)                                                 -- 0.5                                                                               -- 0.5                                                                               -- 0.5                                                                               -- 0.65                                                                              -- 0.65                                 acetanilide                                                                    Mooney Scorch @ 121° C.                                                 t.sub.5, minutes                                                                              17.2                                                                              30.7                                                                              21.3                                                                              28.4                                                                              23.0                                                                              43.6                                                                              17.2                                                                              35.0                                                                              10.0                                                                              29.4                                 % increase in scorch                                                                          -- 78 -- 33 -- 90 -- 103                                                                               -- 194                                  delay                                                                          Stress-Strain @ 153° C.                                                 M.sub.300, MPa 6.9                                                                               6.8                                                                               4.2                                                                               4.6                                                                               3.9                                                                               4.5                                                                               4.2                                                                               4.7                                                                               6.3                                                                               5.6                                  TS, MPa        25.9                                                                              25.4                                                                              21.2                                                                              21.1                                                                              18.1                                                                              21.3                                                                              11.8                                                                              17.6                                                                              24.3                                                                              22.8                                 Elong., %      630                                                                               620                                                                               690                                                                               680                                                                               660                                                                               690                                                                               540                                                                               640                                                                               650                                                                               670                                  __________________________________________________________________________

                                      TABLE 9                                      __________________________________________________________________________                          1  2  3  4  5  6                                          __________________________________________________________________________     NR Masterbatch       158                                                                               158                                                                               158                                                                               158                                                                               158                                                                               158                                        2-mercaptobenzothiazole                                                                             1  1  1  1  1  1                                          Sulfur               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                        2,2-di(phenylthio)-1,3-cyclohexanedione                                                             -- 0.655                                                                             -- -- -- --                                         2,2-di(isopropylthio)-1,3-cyclohexanedione                                                          -- -- 0.52                                                                              -- -- --                                         2,2-di(cyclohexylthio)-1,3-cyclohexanedione                                                         -- -- -- 0.68                                                                              -- --                                         2,2-di(phenylthio)-5,5-dimethyl-1,3-                                                                -- -- -- -- 0.715                                                                             --                                         cyclohexanedione                                                               2,2-di(cyclohexylthio)-5,5-dimethyl-1,3-                                                            -- -- -- -- -- 0.735                                      cyclohexanedione                                                               Mooney Scorch @ 121° C.                                                 t.sub.5, minutes     15.0                                                                              27.4                                                                              24.9                                                                              21.4                                                                              28.7                                                                              20.5                                       % increase in scorch delay                                                                          -- 83 66 43 91 37                                         Rheometer @ 153° C.                                                     t.sub.90 -t.sub.2, min.                                                                             11.8                                                                              9.3                                                                               8.6                                                                               8.0                                                                               8.7                                                                               8.4                                        Stress-Strain @ 153° C.                                                 M.sub.300, MPa       3.2                                                                               3.7                                                                               4.1                                                                               4.2                                                                               4.4                                                                               4.3                                        UTS, MPa             15.0                                                                              19.3                                                                              22.8                                                                              22.8                                                                              21.0                                                                              23.1                                       Elong., %            690                                                                               740                                                                               760                                                                               750                                                                               730                                                                               740                                        __________________________________________________________________________

                                      TABLE 10                                     __________________________________________________________________________                        1  2  3  4  5  6  7                                         __________________________________________________________________________     NR Masterbatch     158                                                                               158                                                                               158                                                                               158                                                                               158                                                                               158                                                                               158                                       2-mercaptobenzothiazole                                                                           1  1  1  1  0.6                                                                               0.6                                                                               0.6                                       Sulfur             2.5                                                                               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                       Diethyl α,α-di(phenylthio)oxalacetate                                                 -- 0.81                                                                              -- -- -- -- --                                        α,α-diphenylthio-2,4-pentanedione                                                     -- -- 0.635                                                                             -- -- -- --                                        Methyl α,α-di(phenylthio)acetoacetate                                                 -- -- -- 0.66                                                                              -- -- --                                        2,2-di(phenylthio)indandione                                                                      -- -- -- -- -- 0.65                                                                              --                                        Diethyl 2,2-di(isopropylthio)oxalacetate                                                          -- -- -- -- -- -- 0.6                                       MOONEY SCORCH @ 121° C.                                                 t.sub.5, minutes   13.0                                                                              27.3                                                                              21.0                                                                              20.0                                                                              7.5                                                                               14.4                                                                              12.4                                      % increase in scorch delay                                                                        -- 110                                                                               62 54 -- 92 65                                        RHEOMETER @ 153° C.                                                     t.sub.90 -t.sub.2, min.                                                                           9.3                                                                               8.8                                                                               8.3                                                                               8.4                                                                               12.4                                                                              9.9                                                                               12.3                                      STRESS-STRAIN @ 153° C.                                                 M.sub.300, MPa     3.4                                                                               3.8                                                                               4.0                                                                               4.1                                                                               3.6                                                                               3.7                                                                               3.3                                       UTS, MPa           17.3                                                                              20.4                                                                              20.8                                                                              20.7                                                                              16.2                                                                              17.9                                                                              17.2                                      Elong., %          700                                                                               720                                                                               700                                                                               690                                                                               660                                                                               680                                                                               730                                       __________________________________________________________________________

                                      TABLE 11                                     __________________________________________________________________________                          1  2  3  4  5  6                                          __________________________________________________________________________     NR Masterbatch       158                                                                               158                                                                               158                                                                               158                                                                               158                                                                               158                                        2-mercaptobenzothiazole                                                                             1  1  1  1  1  1                                          Sulfur               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                        2,2-di(phenylthio)malononitrile                                                                     -- 0.565                                                                             -- -- -- --                                         N-(α,α-di[phenylthio]cyanoacetyl)urethane                                               -- -- 0.745                                                                             -- -- --                                         Ethyl di(phenylthio)cyanoacetate                                                                    -- -- -- 0.66                                                                              -- --                                         2,2-di(phenylthio)-2-cyanoacetamide                                                                 -- -- --    0.60                                                                              --                                         1(α-cyano-α,α-di[phenylthio]acetyl)-                                              -- -- -- -- -- 0.705                                      pyrrolidone                                                                    MOONEY SCORCH @ 121° C.                                                 t.sub.5, minutes     13.0                                                                              32.3                                                                              34.7                                                                              28.5                                                                              24.2                                                                              21.9                                       % increase in scorch delay                                                                          -- 148                                                                               167                                                                               119                                                                               86 68                                         RHEOMETER @ 153° C.                                                     t.sub.90 -t.sub.2, min.                                                                             9.6                                                                               8.8                                                                               9.1                                                                               7.9                                                                               8.3                                                                               7.0                                        STRESS-STRAIN @ 153° C.                                                 M.sub.300, MPa       3.3                                                                               3.6                                                                               3.7                                                                               3.8                                                                               3.9                                                                               4.1                                        UTS, MPa             18.2                                                                              19.5                                                                              19.2                                                                              20.6                                                                              20.3                                                                              22.1                                       Elong., %            710                                                                               720                                                                               710                                                                               730                                                                               720                                                                               720                                        __________________________________________________________________________

                                      TABLE 12                                     __________________________________________________________________________                        1   2   3   4   5                                           __________________________________________________________________________     NR Masterbatch     158                                                         2-mercaptobenzothiazole                                                                           1                                                           Sulfur             2.5                                                         2,2-di(phenylthio)-2-cyanoacetamide                                                               --  0.6 --  --  --                                          2,2-di(isopropylthio)-2-cyanoacetamide                                                            --  --  0.44                                                                               --  --                                          2,2-di(phenylthio)-2-cyanoacetanilide                                                             --  --  --  0.755                                                                              --                                          2,2-di(isopropylthio)-2-cyanoacetanilide                                                          --  --  --  --  0.615                                       MOONEY SCORCH @ 121° C.                                                 t.sub.5, minutes   11.0                                                                               21.5                                                                               23.8                                                                               19.9                                                                               22.7                                        % increase in scorch delay                                                                        --  95  116 81  106                                         STRESS-STRAIN @ 153° C.                                                 M.sub.300, MPa     5.8 5.3 5.0 5.4 5.2                                         UTS, MPa           17.7                                                                               21.1                                                                               20.3                                                                               21.2                                                                               19.0                                        Elong., %          570 640 660 650 630                                         __________________________________________________________________________

                                      TABLE 13                                     __________________________________________________________________________                          1  2  3  4  5  6                                          __________________________________________________________________________     NR Masterbatch       158                                                                               158                                                                               158                                                                               158                                                                               158                                                                               158                                        2-mercaptobenzothiazole                                                                             0.6                                                                               0.6                                                                               0.6                                                                               -- -- --                                         Santocure® NS Accelerator                                                                       -- -- -- 0.6                                                                               0.6                                                                               0.6                                        Sulfur               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                                                               2.5                                        Diethyl 1,1,3,3-tetra(phenylthio)-1,3-                                                              -- 0.57                                                                              -- -- 0.4                                                                               --                                         acetone-dicarboxylate                                                          Diethyl 1,1,3,3-tetra(cyclohexylthio)-                                                              -- -- 0.59                                                                              -- -- 0.4                                        1,3-acetone dicarboxylate                                                      MOONEY SCORCH @ 135° C.                                                 t.sub.5, minutes     5.9                                                                               11.1                                                                              6.5                                                                               13.8                                                                              21.1                                                                              13.7                                       % increase in scorch delay                                                                          -- 88 10 -- 53 --                                         STRESS-STRAIN @ 153° C.                                                 M.sub.300, MPa       5.2                                                                               4.8                                                                               4.7                                                                               8.7                                                                               7.2                                                                               6.8                                        UTS, MPa             13.8                                                                              15.3                                                                              14.2                                                                              27.3                                                                              24.6                                                                              25.2                                       Elong., %            550                                                                               580                                                                               560                                                                               620                                                                               630                                                                               640                                        __________________________________________________________________________

Inhibitors of the invention derived from cyano containing active methylene compounds are illustrated in Tables 11 and 12. The data of Table 11 indicate that the 2,2-di(phenylthio)malononitrile and N-(cyanodi[phenylthio])urethane are especially potent. The data of Table 12 show that derivatives of 2-cyanoacetamide and 2-cyanoacetanilide exhibit good activity and that the isopropyl derivatives are better.

Inhibitors derived from diethyl 1,3-acetonedicarboxylate are illustrated in Table 13. Stocks 1-3 contain 2-mercaptobenzothiazole as accelerator. Stocks 4-6 contain a sulfenamide accelerator. Stocks 1 and 4 are controls containing no inhibitor. The data show that the phenylthio derivative is more potent than the corresponding cyclohexylthio derivative.

Similar results are obtained with inhibitors of the invention in vulcanizable compositions comprising synthetic rubber such as styrene-butadiene rubber.

Although the invention has been illustrated by typical examples, it is not limited thereto. Changes and modifications of the examples of the invention herein chosen for purposes of disclosure can be made which do not constitute departure from the spirit and scope of the invention. 

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A vulcanizable composition comprising sulfur-vulcanizable rubber, sulfur-vulcanizing agent, organic vulcanization accelerating agent and, in an amount effective to inhibit premature vulcanization, an inhibitor of the formulas ##STR19## in which R is alkyl of 1-20 carbon atoms, cycloalkyl of 5-12 carbon atoms, benzyl, α-methylbenzyl, phenyl, or substituted phenyl wherein the substituents are lower alkyl, lower alkoxy, or halo, R₁ is R or anilino or substituted anilino wherein the substituents are lower alkyl, lower alkoxy or halo, and R₂ is phenyl or substituted phenyl as above, ##STR20## in which R is the same as above and X is o-phenylene or straight or branched alkylene of 2-4 chain carbon atoms, ##STR21## in which R is the same as above, R₃ is lower alkyl or lower alkoxy, and R₄ is carboalkoxy of 1-5 carbon atoms or ##STR22## or R₄ may be lower alkyl provided that the vulcanization accelerating agent is 2-mercaptobenzothiazole, and ##STR23## in which R is the same as above and R₅ is cyano, carboalkoxy of 1-5 carbon atoms, substituted carbamoyl of the formula ##STR24## in which R₆ is hydrogen or R, R₇ is R or carboalkoxy of 1-5 carbon atoms, or R₆ and R₇ together is alkylene or oxydialkylene of 4-8 carbon atoms which along with the nitrogen atom forms a heterocycle, or R₅ may be carbamoyl provided that the vulcanization accelerating agent is 2-mercaptobenzothiazole.
 2. The composition of claim 1 in which the rubber is a diene rubber and the vulcanizing agent is elemental sulfur.
 3. The composition of claim 2 comprising, per 100 parts by weight of rubber, 0.5-5 parts by weight sulfur, 0.1-5 parts by weight of accelerating agent, and 0.1 to 2.5 parts by weight of inhibitor.
 4. The composition of claim 3, comprising an inhibitor of Formula (I), in which R is lower alkyl, cycloalkyl of 5-8 carbon atoms, benzyl or phenyl, R₁ is lower alkyl, phenyl or anilino and R₂ is phenyl.
 5. The composition of claim 4 in which R₁ is methyl.
 6. The composition of claim 5 in which R is isopropyl.
 7. The composition of claim 5 in which R is phenyl.
 8. The composition of claim 5 in which R is cyclohexyl.
 9. The composition of claim 4 in which R₁ is anilino.
 10. The composition of claim 4 in which R₁ is phenyl.
 11. The composition of claim 3 comprising an inhibitor of Formula (II) in which R is lower alkyl, cycloalkyl of 5-8 carbon atoms, benzyl or phenyl and X is o-phenylene.
 12. The composition of claim 11 in which R is phenyl.
 13. The composition of claim 11 in which R is isopropyl.
 14. The composition of claim 11 in which R is cyclohexyl.
 15. The composition of claim 3 comprising an inhibitor of Formula (II) in which R is lower alkyl, cycloalkyl of 5-8 carbon atoms, benzyl or phenyl and X is 1,3-propylene.
 16. The composition of claim 15 in which R is phenyl.
 17. The composition of claim 15 in which R is isopropyl.
 18. The composition of claim 15 in which R is cyclohexyl.
 19. The composition of claim 3 comprising an inhibitor of Formula (III) in which R is lower alkyl, cycloalkyl of 5-8 carbon atoms, benzyl or phenyl.
 20. The composition of claim 19 in which R₃ is lower alkoxy and R₄ is lower alkyl.
 21. The composition of claim 19 in which R₃ is lower alkoxy and R₄ is carboalkoxy.
 22. The composition of claim 19 in which R₃ and R₄ are lower alkyl.
 23. The composition of claim 20 in which R₃ is methoxy and R₄ is methyl.
 24. The composition of claim 23 in which R is phenyl.
 25. The composition of claim 21 in which R₃ is ethoxy, R₄ is carbethoxy, and R is isopropyl.
 26. The composition of claim 21 in which R₃ is ethoxy, R₄ is carbethoxy, and R is phenyl.
 27. The composition of claim 22 in which R₃ and R₄ are methyl and R is phenyl.
 28. The composition of claim 3 comprising an inhibitor of Formula (IV).
 29. The composition of claim 28 in which R₅ is cyano.
 30. The composition of claim 29 in which R is phenyl.
 31. The composition of claim 28 in which R₅ is carboalkoxy.
 32. The composition of claim 31 in which R₅ is carbethoxy and R is phenyl.
 33. The composition of claim 28 in which R₅ is carbamoyl.
 34. The composition of claim 33 in which R is phenyl.
 35. The composition of claim 33 in which R is isopropyl.
 36. The composition of claim 28 in which R₅ is substituted carbamoyl and R₆ is hydrogen.
 37. The composition of claim 36 in which R₇ is carboalkoxy.
 38. The composition of claim 37 in which R₇ is carbethoxy and R is phenyl.
 39. The composition of claim 36 in which R₇ is phenylcarbamoyl.
 40. A vulcanizable composition comprising sulfur-vulcanizable diene rubber, and per each 100 parts by weight of said rubber, 0.5-5 parts by weight of sulfur, 0.1-5 parts by weight of 2-mercaptobenzothiazole and 0.1-2.5 parts by weight of an inhibitor of the formulas ##STR25## in which R is alkyl of 1-20 carbon atoms, cycloalkyl of 5-12 carbon atoms, benzyl, α-methylbenzyl, phenyl, or substituted phenyl wherein the substituents are lower alkyl, lower alkoxy, or halo, R₁ is R or anilino or substituted anilino wherein the substituents are lower alkyl, lower alkoxy or halo, and R₂ is phenyl or substituted phenyl as above, ##STR26## in which R is the same as above and X is o-phenylene or straight or branched alkylene of 2-4 chain carbon atoms, ##STR27## in which R is the same as above, R₃ is lower alkyl or lower alkoxy, and R₄ is lower alkyl, carboalkoxy of 1-5 carbon atoms, or ##STR28## and ##STR29## in which R is the same as above and R₅ is cyano, carbalkoxy of 1-5 carbon atoms, carbamoyl, or substituted carbamoyl of the formula ##STR30## in which R₆ is hydrogen or R, R₇ is R or carbalkoxy of 1-5 carbon atoms, or R₆ and R₇ together is alkylene or oxydialkylene of 4-8 carbon atoms which along with the nitrogen atom forms a heterocycle.
 41. The composition of claim 40 comprising an inhibitor of Formula (I) in which R is lower alkyl, cyclohexyl, benzyl or phenyl, R₁ is lower alkyl, phenyl or anilino and R₂ is phenyl.
 42. The composition of claim 40 comprising an inhibitor of Formula (II) in which R is lower alkyl, cyclohexyl, benzyl or phenyl, and X is o-phenylene or 1,3-propylene.
 43. The composition of claim 40 comprising an inhibitor of Formula (III) in which R is lower alkyl, cyclohexyl, benzyl or phenyl.
 44. The composition of claim 43 in which R₃ and R₄ are lower alkyl.
 45. The composition of claim 43 in which R₃ is lower alkoxy and R₄ is lower alkyl.
 46. The composition of claim 43 in which R₃ is lower alkoxy and R₄ is carboalkoxy. 